ESTRO 36 Abstract Book

S882

ESTRO 36 2017

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procedure can be performed if an indisputable evidence

of improvement may be proved and the feasibility in a

clinical workflow is guarantee. In this work IVD performed

with electronic portal imaging device (EPID) was used to

evaluate VMAT and IMRT breast treatment performed with

a new set up and immobilization procedure.

Material and

Methods

IVD with EPID was performed over 32 patients that

underwent an IMRT or VMAT breast plus supraclavicular

treatment. Half of the patients followed the standard set

up procedure (SP) of the department, consisting of a

thermoplastic mask covering the district to be treated,

patient marks over the mask, bolus applied over the mask;

the others followed a new procedure (NP) and were

immobilized with a breast board and a knee support,

patient marks over the skin and bolus applied over the

skin. The accuracy of the treatment was evaluated with a

commercial software (SOFDISO, Best Medical Italy) that

provided two indexes: the ratio R between the

reconstructed (D

iso

) and planned (D

tps

) isocenter dose

(R=D

iso

tps

) which can represent the accuracy of the dose

delivered, and a Pγ% obtained performing a gamma

analysis between the first EPID image and the next ones

acquired immobilized. Three consecutives tests were

scheduled during the first week of treatment and

successively two IVD test per week. The MLC log files of

the treatments delivered where analysed with a

commercial software and compared with the planned

treatment in order to discriminate the deviation coming

from the patient (anatomy and set up), from the deviation

coming from the linac.

Results

Only the IVD test coming from a delivery with the

machines log file in tolerance were considered. The

results of 545 IVD tests obtained over 32 patients were

reported in Table 1. Every treatment performed with IMRT

and VMAT resulted in 100% of the patients with R and Pγ

indexes in tolerance as for SP as for NP. The percentage

of Pγ index in tolerance as for VMAT as for IMRT increased

with NP. A 10% of, off tolerance tests persisted. The IVD

tests off tolerance were reported in the acceptable

threshold before the next fraction.

Discussion:

the new

immobilization procedure enabled a direct localization of

the patient skin and of the bolus positioned over it. The

use of the mould mask, positioned over the patient’s lead

to a non-direct evaluation of the patient rotation and

accommodation inside it. The beam can lack of

reproducibility if considering its path: air gap between the

bolus, the mask and the patient skin not considered in the

treatment planning. This aspect is moreover important for

IMRT treatment where for some beam entry this situation

can be more evident.

Conclusion

IVD in is a powerful tool that can be helpful in the

validation of new set up and immobilization procedures.

EP-1648 Thermoplastic mask dependency with

interfractional uncertainties for head and neck VMAT

treatments

E.M. Ambroa Rey

, R. Gómez Pardos

, D. Navarro

Giménez

, A. Ramirez Muñoz

, M. Colomer Truyols

Consorci Sanitari de Terrassa, Medical Physics Unit-

Radiation Oncology Department, Terrassa, Spain

Purpose or Objective

Volumetric-modulated arc therapy (VMAT) techniques

have the ability to deliver a highly conformal dose

distribution to the target and high dose gradient at the

interface between the tumor and the normal tissues,

decreasing the irradiated volume and sparing OARs.

However inaccurate alignment of the radiation beam with

the patient can lead to critical organs to receive an

unwanted high dose or the tumor to receive a reduced

dose producing a loss in tumor control. Radiation therapy

for head-and-neck (H&N) cancer requires a reliable

immobilization

for

accurate

treatment.

The purpose of this study is to establish the interfractional

setup error for VMAT H&N patients, using a kilovoltage

cone beam CT (CBCT) and a robotic treatment couch

(HexaPOD) for accurate patient positioning in six degrees

of freedom and analyze the differences between two

types of thermoplastic masks (Head mask (H) and Head

and Shoulder (HS) mask).

Material and Methods

Dataset for a total of 945 CBCT scans were obtained from

30 patients (13 with H mask and 17 with HS mask). For

each fraction, patients were placed on the HexaPOD

robotic couch and a CBCT was performed. For

interfraction accuracy evaluation, the daily variations of

the three principal axes (X, Y and Z) and three rotational

movements (pitch, roll, and yaw) were extracted. Also,

the type of thermoplastic mask was recorded. The

following parameters were calculated: the mean of the

setup corrections (M), the standard deviation (random

error, σi), the group random error (σ) defined as the mean

of all the individual patient random error σi and the

systematic group error (∑) defined as the standard

deviation of all the means measured for each patient.

Results

Results are shown in Table I. The overall mean

displacements are larger for the HS mask patients than for

the H mask. Also a trend toward a positive antero-

posterior direction (1.19 mm) was observed in

translational displacements for the HS mask patients. This

could be explained taking into account the differences

between the simulation and treatment couches as well as

the accuracy of the room lasers. The remaining error

components

did

not

show

any

trend.

For the rotational directions, the bigger error was in the

pitch direction for both the H mask (0.76) and HS mask

patients (0.85).

Regarding the systematic group error, we have found a

larger error for the HS mask patients (2.34) again toward

a positive antero-posterior direction. The random group

error shows the same behavior as well.

Conclusion